Metal electrodes product selectivity

Selective silylation of polychloromethanes using reactive metal electrodes such as zinc and magnesium has also been reported as shown in Scheme 37 [76, 77]. The electroreduction of carbon tetrachloride and chloroform in the presence of chlorotrimethylsilane affords the monosilylated and disilylated products. The product selectivity seems to depend upon the electrode material. 

Atobe and Nonaka [67] have used a 20 kHz (titanium-alloy) sonic horn as the electrode (called sonoelectrode) for electroreductions of various benzaldehyde derivatives. This they did after insulating the submerged metal part of the horn-barrel with heat-shrink plastic. They found an improvement in current efficiency with insonation, but in addition noted some change in product selectivity towards one-electron-per-mole-cule products. Although the authors quote enhanced mass transfer across the electrode interface as the origin of the sonoelectrochemical trend towards products from the lesser amount of electrons per substrate molecule, the involvement of surface species on the reactive electrode provides a complication. 

First, C02 reduction at metal electrodes in both aqueous and nonaqueous media, as well as in systems coupled with electron-mediating complexes are detailed. The faradaic efficiency of such a system can be used as a measure of efficiency and selectivity. For a specific, electrochemically generated product, the faradaic efficiency is the ratio of the actual and theoretical amounts of product formed within the same time interval, based on charge passed. An efficient and selective system will lead to a 100% faradaic yield for a single product in other words, all of the charge passed in the system has gone into the production of that product. 

Amperometric biosensors combine the specificity and selectivity of biological sensing components with the analytical power of electrochemistry. Because of the use of metal electrodes which can be deposided on nearly all substrates, thin and thick film technology is the method of first choice. The limitation which occurs with silicon device technology is no longer decisive and biosensor production is not restricted to silicon production lines. 

We demonstrated that the adsorption strength between CO and electrode metals relates closely with the product selectivity in CO and COj electroreduction, and that moderate strength of CO adsorption on Cu electrode is suitable for production of hydrocarbons.[11] The present results of IR spectroscopy evidently confirm the validity of our hypothesis. 

It is well known that trace amount of impurities will interfere with surface process in electrochemical reactions on the electrodes as well as any heterogeneous interfacial reactions. The electrochemical reduction of CO2 is also a surface process, and is naturally sensitive to the cleanliness of the electrode. Any surface contamination will lead to variation of electrocatalytic property of the electrode. The product selectivity of metal electrode is often severely affected by the presence of extremely small amount of adatoms on the surface." The surface contamination is sometimes a source of many controversial experimental results. 

The product distribution in CO2 reduction varies widely, depending primarily on the electrode metals and the electrolyte solutions used for the reaction. Various reactions proceed simultaneously in parallel on the electrode surface. The electrode provides the site of the reaction, and the product selectivity in CO- reduction is affected by whether or not the reactants and other related species are adsorbed. The selectivity also depends on the strength of the adsorption, if any species is adsorbed on the surface. The electrolyte solution determines the concentration and the stnbilitv of the reac-... 

Hori and his coworkers carried out CO2 reduction at various metal electrodes in constant current electrolysis at 5 mA cm in 0.5 M KHCO3 aqueous solution purified with preelectrolysis. They applied full chemical analysis of the products and studied the faradaic balance. They revealed that CO2 reduction in aqueous media yields measurable amount of CO, CH4 and other hydrocarbons as well as formic acid at ambient temperature and pressure in a reproducible way, and the product selectivity depends greatly on the metal electrodes. The product distribution is tabulated in terms of faradaic efficiency with the current densities in Table 3, which contains the results revised in their later publications. The product selectivity is greatly affected by the purity of the electrode metals as well as that of the electrolyte solution. Tire results above were confirmed later by other workers. "" ... 

Metal electrodes are divided into 4 groups in accordance with the product selectivity indicated in Table 3. Pb, Hg, In. Sn, Cd, Tl, and Bi give formate ion as the major product. Au. Ag, Zn. Pd, and Ga, the 2nd group metals, form CO as the major product. Cu electrode produces CH4, C2H4 and alcohols in quantitatively reproducible amounts. The 4th metals, Ni, Fe, Pt, and Ti. do not practically give product from CO2 reduction continuously, but hydrogen evolution occurs. The classification of metals appears loosely related with that in the periodic table. However, the correlation is not very strong, and the classification such as d metals and sp metals does not appear relevant. More details of the electrocatalytic properties of individual metal electrodes will be discussed later. 

Table 4 gives a list of electrode metals used in research works of electrochemical reduction of CO2 published roughly after 1970. The electrode metals appear in accordance with the product selectivity mentioned above. Metals with low activity in CO2 reduction at ambient pressure are divided into two groups metals with which activity appears under elevated pressure, and ones otherwise. 

Methanol is a nonaqueous solvent, but normally classified as a protic solvent with many similarities with water. The product selectivity of CO2 reduction depends on the electrode metal in me-... 

Classification of Electrode Metals in Accordance with Product Selectivity in CO2 Reduction in Aqueous and Nonaqueous Electrolyte (PC). Reproduced from Ref. 23, Copyright (1994) with permission from Elsevier... 

The TVD curves of selected amino acids were determined by Contarini and Wendlandt (121). A comparison of the TVD and DSC peak temperature is shown in Table 11.8. The TVD peak temperatures are somewhat higher than those obtained by DSC. Obviously, the kinetics of the electrodedecomposition produces) reaction are different from those of the decomposition reaction. These electrode reactions probably involve one or more diffusion steps between the electrode surface and the amino acid or amino acid decomposition produces), which would be different from the decomposition kinetics themselves. The leading edge of the TVD curve peaks is reproducible to within +1-2%. However, after the peak maximum temperature is attained, the reproducibility falls to within +20% in some cases. This is related to the electrode-amino acid decomposition products interface, which, due to the nature of the reaction, would not be expected to be reproducible. The trailing edge portion of the curve also consists of several shoulder peaks that may be related to the consecutive and/or concurrent reactions previously described in the DSC curves. These reactions could produce decomposition products that would react with the aluminum metal electrode surface. 

Electrosorption and Reduction of CO2. - Nowadays the electrochemical reduction of carbon dioxide to useful organic materials and fuels is an important topic with both theoretical ° and practical interest. The CO2 reduction at metal electrodes in aqueous media yields CO, HCOO, CH4, C2H4, and alcohols. The metal electrodes that show activity in CO2 reduction can be divided according to the product selectivity into the following groups ... 

Watanabe and coworkers have found that alloy electrodes are often more efficient than electrodes made of pure metals. They have studies the reduction of CO on copper alloys with nickel, tin, lead, zinc cadmium and mercury [24]. The alloys were all produced by electroplating from mixtures of the metal ions in solution. In the cases of nickel, tin, lead and zinc alloys, current densities and faradaic efficiencies were all found to be greater than those of the pure components. Also product selectivity was found to be a function of the metal alloyed with copper. 

The effect of the electrocatalyst composition was studied as well. A series of Ag electrode cells was modified by incorporating a second metal or metal oxide. The results for inclusion of Sm, Ho, Li/Mg, and Bi, along with the naked Ag anode cell are summarized in Table 4. These cells all had about the same current and methane flow rate, so that the effect of the residence time and CH4/O2 ratio was minimized. The methane conversions were clustered between 2.9% and 4.3%, but the selectivity to C2+ product ranged from 9.3% for Sm to 70.3% for Bi. Thus, there is a dramatic effect of catalytic metal on the selectivity to higher hydrocarbons. 

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